The invention relates to a press drive for a press. The press drive comprises a knee lever gear. The knee lever gear is driven by an eccentric drive that can also be referred to as a crank drive. The knee lever gear couples the eccentric drive with a ram of the press, so that the driving motion of the eccentric of the eccentric drive effects a linear motion of the ram in stroke direction.
Presses comprising knee lever gears have been generally known. Publication DE 10 2005 001 878 B3 discloses a press drive comprising a knee lever gear, wherein the ram of the press is associated with an ancillary drive. This ancillary drive is disposed to ensure sufficient ram force, in particular, in certain articulation angle regions of the lever of the knee lever gear.
Publication DE 10 2007 022 715 A1 describes a knee lever gear comprising two knee lever arrangements that can be actuated via a shared linear drive that acts on the hinged joints of the two knee lever gears. When one knee lever gear is driven via the linear drive, the transmission function relative to the extended position of the knee lever gear is symmetrical, i.e., the ram performs the same movement, irrespective of whether the hinged joint is articulated to the one or the other side, starting from the extended position.
From publication DE 21 27 289 A an adjustable knee lever drive is known. A main eccentric drives a main connecting rod that represents the first lever of the knee lever gear, said first lever being connected to the ram via a second lever. Via an auxiliary connecting rod, an auxiliary eccentric acts on one arm of a two-arm lever. The other arm of this two-arm lever is coupled with the hinged joint. The linkage points of the auxiliary connecting rod on the two-arm lever, as well as of a drive rod between the two-arm lever and the hinged joint, are adjustable. This measure is intended to allow an adjustment of the impact velocity of the ram on the tool, the travel of the ram stroke, the stroke length and the position of the lower return point of the ram.
Publication DE 198 46 951 A1 describes another press comprising a knee lever gear. The first lever of the knee lever gear is supported by the press frame, while the other lever is connected to the ram. The connection between these two levers is accomplished via a triangular control arm, so that the first lever and the second lever are supported at a distance from each other by the triangular control arm. Furthermore, the triangular control arm is connected to an eccentric drive by means of a connecting rod. The length of the connecting rod is adjustable. If the knee lever gear oscillates through an extended position, the ram is briefly moved twice in succession through a lower return point due to the kinematics of the arrangement. The location of these two lower return points differs relative to a reference point on the press frame in stroke direction. If the knee lever gear does not oscillate through its extended position, a common, approximately sinusoidal, progression of the ram position is achieved.
The disadvantage of this arrangement is the differing position of the two lower return points when the knee lever gear is moved through its extended position. Also, in many cases a changing connecting rod length is undesirable. The length change changes the course of movement of the end of the connecting rod that is connected to the hinged joint. Furthermore, an arrangement for changing the length of the connecting rod, in particular if such a change is to be accomplished by an actuating drive, is complex considering the design and considerably increases the moved mass of the connecting rod.
Referring to these described press drives, it can be viewed as an object of the present invention to provide a press drive of a simple design that, nevertheless, allows different operating modes depending on the task to be performed by the press.